If she has what's usually known as a 'mophead' hydrangea, she shouldn't be deadheading - these should be left in place until the following spring for added protection of buds waiting for next year over winter. Pruning should be restricted to merely removing obviously dead wood in about April, as growth begins.
As its in a pot, its possible she didn't keep up sufficiently well with watering - dryness at the roots will reduce flowering too. Growmore would not have prevented flowering taking place, but I do wonder what size pot she's used - it sounds as if she's used a giant pot for an initially small plant.

Have to agree with the pruning advice, but I have also heard that you should not feed hydrangeas with any rich nitrogen based feeds, like growmore, as they tend to put on leaf but flower less. Lack of water, as bamboo says, will also considerably reduce flowering. Perhaps in the spring it just needs some iron, potassium and magnesium? There are special hydrangea feeds available at garden centres.

I was assuming the lady only used Growmore once in Spring, and possibly again 6 weeks later, Avkg47, which is fine for a potted hydrangea. Being in a pot, it will need something occasionally.
Most likely explanation is pruning off growth buds when she pruned.

Thank you for your answers, Bamboo & Avkg47. I didn't think to ask her about watering at the time, I'll have to make a note & ask her again. I didn't know Growmore was Nitrogen rich, I've never used it myself. Iron, potassium and magnesium would all be supplied in the tomato/rose fertilizer I recommended she use.

She told me she hadn't pruned them back much & that she didn't do it until she saw a neighbour on her street cutting his back. It seems he has big bushes. I have never grown or pruned Hydrangeas so have no 1st hand experience but I did know about not cutting off the dead heads in winter as they protect next year's flower buds, something I asked her about.

Growmore isn't nitrogen rich - it's a balanced feed with a ratio (NPK) of 7-7-7, sometimes 6-6-6. Its actually better to use something like Vitax Q4 though because that has trace elements which Growmore doesn't, though in a pot, I'd be inclined to use Miracle Gro multi purpose, which can be watered on. Tomato food, on the other hand, is not at all balanced in the same way, having a very high potassium level - typical tomato food will have an NPK of 4-4-12, but may even be 6-6-20, so not so good for leafy shrubs.

Growmore is not especially nitrogen rich, as far as grass is concerened!, as both you and bamboo state, but If the plants have been pruned properly, and watered regularly - hydrangeas do like a moist base - then the answer has to be in the feed they are receiving. As long as the pot is not too small, then the bigger the pot the better, along with a siting that is not too open to strong light and cold wind, and slightly shaded. Hydrangeas are mineral feeders - ericaceous varieties need sequestered iron to convert feed they can use. Again, I recommend a visit to your local garden centre to see what hydrangea feeds they have and see if that makes a difference.

What do you mean by 'ericaceous varieties' in regard to Hydrangea, Avkg47?

Avkg47 - you've not got back to this thread yet, but for the sake of increasing accuraate knowledge for members on the site, can I just point out that there are no 'ericaceous' varieties of hydrangea - all will grow happily in alkaline soil, nor do they need sequestered iron to metabolize feed.
Perhaps what you mean is that H. macrophylla varieties have flower colours which vary according to the ph of the soil in which they are growing - acid soil gives blue flowers, alkaline pink, neutral ph gives that rather unpleasant shade of lilacy pink. The ph is only affecting the flower colour though, and does not affect the health of the plant, nor its ability to take up nutrients.
Hydrangea macrophylla varieties are very useful when you move to a new area and don't know the ph of your soil, they work like a ph meter - you'll soon find out when the flowers arrive...

I wrote a reply on here yesterday but I don't see it here! I don't know what happened. Perhaps I thought I pressed the "Add Comment" button but didn't actually do it!

I must agree with Bamboo as to there being NO 'Ericaceous' varieties of Hydrangea. The acid/alkaline (ph) levels of the soil they are planted in only affects the colour of the flowers NOT the plants ability to take up nutrients. They will live perfectly happy whatever the soil ph might be & acid soil does not affect their ability to grow & neither do they need sequestered iron, unlike other plants that will die if not given acid conditions or sequestered iron when planted in alkaline soil.

I've found a website that seems to give fairly accurate information on Hydrangeas not flowering. The site is called "Gardening Know-how".

Reasons And Fixes For A Hydrangea Not Blooming

http://www.gardeningknowhow.com/flower/hydrangea-not-blooming.htm

Here is another webpage that has some good advice on how to prune Hydrangeas so that they flower every year.

http://www.hydrangeashydrangeas.com/pruning.html