Friend of CL opponent scheduling Friendlies just before our match

[HeavensAAA]

Theoretically, there should be a restriction that starting time of matches has to be 15 minutes from each other.

You could not ‘block’ a friendly being arranged (though I never experienced anyone trying to arrange friendly with me whilst they are not within the friend list).

An alternative solution is:
1) Cancel that unwanted friendly
2) Then arrange a friendly to start some 20 minutes, say 10.30, before your scheduled CL game
3) Then at around 10.25, you cancel that friendly

At least you could ‘block’ others in trying to interfere with your match preparation.