Wedding gift?

I'd do as they ask.

If you visit them after the wedding, take a nice bunch of flowers or something similar. After all, you often take flowers when you visit someone whether or not they've recently got married.

I'd go and have a lovely time!
How lovely that there are no stipulations.

Maybe you could take them out for a day or a meal once the wedding has been done and dusted?

I too would do as asked. They clearly don’t want presents. Make a donation to a charity they would approve of if you want to give something.

Don't take anything - you'd make them feel uncomfortable which is just what you don't want ! I got married in my 60's after living together for 5 years - we said the same thing on the invitations as your friend. I'd have been highly embarrassed if anyone had turned up with a gift . Just go and wish them well .

You could buy them an Oxfam goat or similar if you wanted to.

A donation to some cause they would appreciate.

A voucher for a meal or two in a restaurant (most restaurants produce nice cards to present to the recipient)

A subscription to a magazine or organisation.

If you know their preferences, likes and dislikes, that's your inspiration.

A good bottle of champagne.

We were married 4 years ago, after living together for 40 years. We stipulated no gifts, because we really didn't need or want anything. It was much more important to have the people there that we care about.
You are obviously a thoughtful friend and they want you to be part of the event. Enjoy the occasion but don't feel embarrassed about not taking a gift.
I would suggest catching up with the couple at a later date and taking them something small. Personally I love receiving flowers or fizz and OH loves receiving chocolates and red wine...

Do as they ask, if they had wanted gifts they would not have stipulated no gifts, enjoy the wedding.