Yes, probably - usually flowers late spring, but you might find that florists can still get hold of it.

I heard that the lilies of the valley that the Duchess of Cambridge (or Kate Middleton as we used to know her) had in her bouquet for her wedding on 29th April were grown just a few miles from me in Cornwall. The story is that the plants and flowers had to be refrigerated to stop them from "going over" too soon. I really don't know if you could hold them back another month or two til June. Probably not - you could have the wedding earlier....? ;-)

I have lilies of the valley in my garden (hundreds of them) and they flowered in May.

When I got married (many moons ago) I requested lilies of the valley but the florist said they wouldn't be able to supply real ones (I was getting married in August) but instead they could put 'silk' ones in the bouquet. You couldn't actually tell they were fake, so maybe this is an option for her.

i too had silk ones as early april was too soon.

Yes you can do it. Years ago an old gardener told me they could be made to flower at any time of year by refrigerating them at the right time. Unfortunately I can't remember the details. But
have a look at http://www.superfloralretailing.com/pdf/CutFlower0408.pdf (or google lily of the valley refrigerating)