The 'Glee' actor was hoping to surprise his co-star beau for her 27th birthday, which is on August 29 , and had been making arrangements before he was found dead in a hotel room in Vancouver, Canada, on July 13.
A source told Us Weekly magazine: ''She [Lea] didn't know what [he was planning] but she thought it could be a party and possibly a trip somewhere.''
The pair had been dating since early 2012 and were hoping to move in together before Cory died from an accidental alcohol and heroin overdose.
The source added: ''Lea and Cory were planning on moving in together after he got back from Vancouver. [They] weren't talking engagement or marriage yet, but it was heading that way.
''The first step was that Cory was going to move in [to Lea's Los Angeles home] completely. He was already there all the time.''
The 26-year-old actress, who was in Mexico when she heard the tragic news, supported Cory, 31, when he entered rehab earlier this year to seek treatment for his addiction to drugs for the second time and believed he was strong enough to fully recover.
Despite all they had been through together they were very happy and in love before he passed away.
The source added: ''They were 100 per cent happy and together and in love at the time of his death.''
Take a look back at October's inaugural event.
The film is expected to continue without Mendes' involvement.