Here's my what if regarding about your opinion.
What if you joined him and the tide's changed, he started on losing and you bet, big to bigger bets having a thought that he could win the next round, but he didn't, right after that, I am sure you will put the blame on him and going to roast him like he forced you to join him. The advantage of joining forces with someone you think good at gambling is a nice move, but you also have to consider that there are times that he is going to lose and that times might be your wrong move and the result is too far from what you are expecting.
By saying this, I recommend not to do this unless you are ready for the consequences that possible to happen, so there would be no blaming scenarios in the end, that's very unprofessional and immature.