Just click on the reject button. I have had numerous friend requests from people I do not know and I always reject them. They are generally from people who want to mess your game up by supporting your opponent (as in your case) or by messing with your game plan by playing friendlies just before your next big game. Personally, I do not need numerous friends as you cannot talk to them online and you can only get 1 booster per day from friends anyway and as per helping by watching a game, (full attendance 5%, watching game 8% then 1 friend 2%) equals maximum of 15%, so why do you need so many friends. Beware of dubious friend requests they are not necessarily in good faith.