I've had Hearts supporting mates come with me to Firhill. I've never gone to Tynecastle though - but my reasoning is that Thistle need extra people through the turnstyle, I've had Rangers, Celtic, Ayr United, Clyde, Falkirk, Fulham fans etc and last home game I had two Hammarby(Sweden) fans at the game and would repay the favour by visiting those stadiums too(apart from Old Firm).
End of the day if getting money through the gate for Thistle and getting to see a game of football, if I pay to go to another stadium and watch someone elses team I am still watching the game I love.