Not really a rose person, I am sure there are loads of people on here who are. I bought some climbers (expensive but worth it) and one had a really long straight shoot appear which I decided was a sucker, hacked it off only to discover that that particular shoot was the one with the label on it - so definitely not a sucker!
Yours looks the same as the other flowers and you could incorporate it into the bush when pruning, but if its spreading too far out - just cut it off!
Now we will see what the real knowledgeable rose people say.

Thanks, Honeysuckle! I don't think it can be a sucker with such a lovely rose on it - perhaps the bark mulch has been put on really thickly.

A sucker has 7 'lobes' (or whatever they're called) on it's leaves - like a wild rose. A 'non-sucker' (or whatever it's called) has 5 'lobes' (or whatever they're called)
Hope you can understand that lol :D

The suckers come from below the graft, from the rootstock

Thanks Hywel and Pamg. Before it flowered, we assumed it was coming from below the graft, because that's definitely how it appears, but it has only got 5 lobes (if that's what they're called!). If it's flowering like that, though, the same as the shoots above the graft, I think she'll just prune it normally. I'll suggest she removes some of the mulch and has a proper look.

That sounds right Mel, technically I think that the graft shouldn't be covered anyway as it does encourage suckers

It is just a nice new strong shoot, worth keeping.

Thanks Lizziebee! All things considered, that's what I think too.