Still unclear as to whether I can head to T3 lounges arriving T3 but departing T5 - I have a 7 hour layover.

Last month I arrived T5, transited to T3 for my departure flight. However, I had invited the chap seated beside me to join me in the lounge at T3 on the understanding he was also departing T3. As we stood in security before entering departure area in T3 he confirmed in fact he was departing T5. He passed through before security showing his BP, went through security and joined me in the BA T3 lounge. No questions asked. Do not know what became of him as 2 hours before his T5 departure I recommended he start on his return journey to T5...when I left him he had an hour to get there...I doubt he made. He seemed to have been enamoured by the offerings from the buffet.