there a rose called golden wedding. But it might be prudent to ask if they'd like an plant to celebrate their anniversary. The reason I say this is family bought hubby and myself a rose for our 25th and neither of us are that keen on roses. Though I do have a few.
Perhaps plant vouchers so they can chose what they'd like and have an enjoyable day out too.

There's also a lovely rose called Golden Celebration which is available from David Austin roses I have it,a lovely rose, or Cornus Controversa 'Variegata'(wedding cake tree) now if someone bought me one of those I'd be a very happy bunny:-)

It's not the gift but the thought that counts and your consideration of them as friends on that occasion is a very fine gift indeed no matter what you choose to represent it with.

As well as Rosa 'Golden Wedding' already mentioned there is Hosta 'Golden Anniversary' and Fuchsia 'Golden Anniversary'. Some sort of rose bowl or vase or outside container could also be given and there are some great outdoor clocks too. But I agree that a garden voucher can be a great gift and then I thought of a couple of tickets to an open garden?

It may be the thought that counts Loosestrife but if someone gave Bulba and I a rose it wouldn't be finding a home in our garden… The gift of a shrub needs to be one that fits in with the recipients ideas for their garden. Strange nick-nacks can simply go to a charity shop a shrub is a bit more of a challenge.

Charity shop? Thanks Moon Grow, I finally know what to do with that figure of Budda with a clock in its belly that I got for XMas.

Many thanks for the replies, they are much appreciated.