My cousin who's mentally challenged has a crush on a woman who isn't, should I let her know?

No.

1. She will prolly work it out eventually.
2. He currently has a fantasty - which is nice to have
3. It's not really your business.
4. Sometimes you have to let life happen.

Ask yourself this: if you were single would you want someone else to tell the person you liked that you had a crush on them?

If the woman is in a certain age group she is primarily interested in finding a good lover. If your cousin can't meet her sexual needs and desires she will quickly dump him.

Try not to interfere too much....you never know....🤷

lol well, it's kinda unlikely that einstein's cousin was anywhere near as smart as he was when they got married, sooo..who's to say?
tho lol just the same, eh..

She’ll see it for herself

Why would you do that

Stay out of it. Not your business.

As long as they are both adults you don’t say sh*t unless asked just like any other adult.