I would do as others suggested and recommend your friend throw £5 into an envelope and leave it in the basket, thats, if she decided to go. My husband is approaching 70 next month and we are just having a small family dinner, and then going to a favourite hotel for a couple of days. He insists on no party or gifts from anyone . We are very fortunate to have all we need, perhaps not all we would want, but that's a different story !!! My friend is also celebrating a milestone birthday next month, and instead of a party is hosting a Macmillan coffee morning , with donations in lieu of gifts. Ive already bought a small gift anyway, but will be happy to make a donation.
Regarding attending events, a friend pointed out to me one time, when I was fretting about whether or not I was obliged to attend a party " Its an invitation, not an obligation " Wise words.