Have you spoken to the lover about the stealing? I think it's important that you confront this person and insist it stops OR you will need to speak to your mom about it. In the meantime, find a way to secure your valuables - locking luggage, a travel safe, etc - whatever you can do to keep your things safe.
When speaking with the lover I would make it clear to them that this is a serious issue and the authorities may be alerted should the stealing continue.
I would talk to your mom as a last resort. Should you not be able to resolve this - then talk to her. She should be made aware of the situation - but it might be best to discuss it once it's been resolved. This way she does not have to choose between you. It is certainly something she should know about though for her own safety as well.
Good luck. I'm so sorry this is happening to you.