Quite stable?
BTC/USD pretty regularly has 15-25% intraday moves, and has ranged from 650 to 1350 (>100%) over the past 5 months. It would be a trivial programming effort to allow the user to view his/her balance in the currency of his/her choice, would it not?
How about you figure it out yourself? This is a BTC site, if you care about the fiat value of BTC then you should do the math. BTC sites generally don't have this implemented as the overwhelming majority of us only care about BTC wins/losses.
BTC is the currency of choice.
The overwhelming majority of sports bettors (Fairlay's target audience) don't follow the fluctuations of BTC any more than they have to. That's not changing any time soon. So assuming Fairlay is interested in expanding its user base beyond BTC enthusiasts, they should be doing everything they can to attract traditional sports bettors. 30 minutes of programming time to make users lives easier doesn't seem like a lot to ask.