Crazy!!! Much needed Advice

My ex girl friends best friend is staying with me this summer. She is no longer in touch with my ex and her and i have become close friends. She lives in Slovakia but i got her a summer job her in LA. We share my small studio apt and do everything together. We get along great. Where both libras and im really attracted to her. Yes... Yes.. I do everything for her with the best intensions and treat her really well. Though she says where great friends and doesnt see me as a boyfriend there is strong tension off and on between us. Last night we got stoned and went to bed. She started to masterbate like always and shes not to far from me i started to watch then i laid back down andi started to masterbate aswell. Then i knew she was watching me but i acted as if i had no idea she was awake and kept going. When i would finish she would lay back down and she would start again. When i heard her stop i would go again. We both knew what we where doing and knew what each other was up to. I must have gone about 7 rounds knowing she was watching. We had a great day today with each other strictly platonic and didnt bring up last night.
My question is knowing this is short term for the summer what are my chances of getting with her?? Next time shes getting off should i just join in??? How do i communicate this?? After seeing friends with benefits today is there now some forshadowing and how do i initiate this???

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This type of situation cannot be acted upon unless with much caution, by you both sitting down and have a frank discussion about it over coffee. If you go jumpin onto her while she is in the act, you may get negative results and the police later knocking at your door questioning you about a sexual assault.