Greetings @phoenix_dawn!
Others have given some wonderful advice as well as spellwork suggestions to help your friend. I agree with what several others have said (as well as the advice you yourself gave to your friend) that banishing negative energy/entities is probably the best and safest way forward.
If it is indeed a curse, why not try a Return to Sender Spell? This type of spell sends the bad energy back to the caster and protects your friend in the process.
If your friend still insists on trying to trap the negative energy, I really like the spell that @moonshadow shared 
There is also the possibility of trying to harness it somehow- perhaps by making a contract with it (like a Familiar Spirit). It goes without saying that handling dark forces is risky business- I’d recommend trying something like this only as a last resort.
Sending good thoughts to you and to your friend- she is lucky to have you on her side helping her out! 
Blessed be 