@Pondering As @saltandpepper has already said sticking to boundaries is very important. Absolutely do not give in and enable them to continue spending without consequence, especially when it is your money. They may get upset with you, but they have to take responsibility for their own illness, ie take their medication, learn their triggers, learn boundaries and learn how to stick to them.
I have bipolar 1 and get mania and the "urge to splurge" also so I can understand this from both sides. Sometimes we need tough love, we don't like hearing it, but it is absolutely necessary for our recovery and treatment. And sometimes tough love is the other person saying No and is us falling on our butts and learning the responsibility of looking after ourselves, by staying on our medication, by asking other people to help us when we are in danger of spiraling out of control, etc.
It is perfectly fine to want to help this person and is is perfectly understandable to not want to upset them. But there is also a time when giving them some cold hard facts and setting some tough boundaries and sticking to them are necessary. You absolutely do not have to justify yourself. You have to look after yourself and your finances too.
I hope this helps. xx
BB 🐰💙