I would do it this way (because it eliminates one security and passport check during the airside T5-T3 movement)
1. clear immigration at T5 and then do the landside transfer to T3*
2. Meet friend landside (but note not that many cafes etc landside but check the terminal maps on the LHR website for details)*
3. After meeting your friends clear security at T3 to take flight to HEL
* Or you could meet your friends at T5 and then lansdide transfer to T3