- Before few months he could have the option (as a friend) to arrange a friendly match vs your team - but now anyone can do it through associations.
- He can see as a friend also your games in the C.L. or the Cup.
So if you 're switching formations from game to game, he can spy on every game you played so far.
- If you 're playing together at his home, he can see your C.L. or Cup game that day and change the time.
For example if you 're playing together at 16.00 and you have a C.L. game at 19.00, he might see it and arrange your League game at 22.00 so you 'll have some tired from previous match players.
Of course you can do the same.
- If you deny and he also made a friend request in a third League competitor (who accepted), he might favor him (against you) in their between game that he doesn't care.

In generals I would say yes, why not.
And watch -support him in his C.L. or Cup games (and "study" him too)