Guaranteed Profits?

Good Morning, 

A person i know is in what seems like a very profitable situation. He has two sportsbooks who both offer him 10% back on losing weeks, no matter what. If he wins $1000 week 1, then loses $1000 week 2, he receives $100 back for his week 2 losses. 

My question to you all is: How can he guarantee himself profitable weeks? He can bet any sport and horses, futures, props, parlays and teasers. 

There are times where the two books have different lines but this is rare and doesn’t happen often enough to secure profits week in and week out. 

If there’s any mathematician or veteran sports gambler who could offer some help, my pal would surely appreciate it.

Thanks all

@AceRothstein27

 Sounds like they would have to bet the opposite side of the same game/prop in a situation where both sides were some how greater than -109 (-108,-107...)