Possible Options:

1. Handshake/Approval
When you send a request for a friendly match the opposing player must agree first in order for the match to commence. This option will only work efficiently if an alert or a message is sent to the opposing player saying that he has been challenged to a match and that he/she has 1 hour to respond otherwise the match will commence regardless of his/her actions.

2. Preset Option
The user can choose from 0 to 4 friendly matches on a daily basis. This means a selection menu with 1, 2, 3, 4 friendly matches should be enabled for that specific date. AND what this means is that if you select [2], that means that you can only be challenged to a friendly match 0, 1 or 2 times in that time period.

I assume these options wouldnt be that hard to integrate and implement as part of the game via scheduled updates.