If Fenn uses this ability to give 2 movement points to another hero, do they interrupt to use those points or do they have to wait for their own activation? We played it the second way yesterday, placing the exhausted ability card next to the relevant hero card as a reminder.

I'm away from my rulebooks right now, but I know that if you gain movement points and it's not your turn (or not your activation), you stop the current activation and immediately resolve the movement. Then return to the current activation.

I'm sure someone will quote the exact rule soon.

You're right. Page 20 of the RRG, bullet point 4 under Movement Points. They really should have made this clear on the card though: "If you grant movement points to another hero then they must interrupt immediately to use them" or something like that.

Yeah, but the cards are so small with limited space.

For those curious, the rule under Movement Points states:

If a figure gains movement points when it is not its activation,

those movement points must be spent immediately as an

interrupt or be lost.

and just to clarify, if Fenn uses the ability on himself, he can use the movement point at any time during his activasion

and just to clarify, if Fenn uses the ability on himself, he can use the movement point at any time during his activasion

Yes, i think that is correct, since it happens within an activation.

Your movement pool is 0 and gets 2 added to use at your leisure.