There is some duplication with the techniques mentioned in this question. However, if Matthew is travelling a lot in that area, chances are he travels with the same airline a lot. So probably his best option is to make sure he always travels with the same airline or alliance and uses their frequent-flyer programme. Sooner or later he'll get upgraded to an elite level and start getting lounge entry for free (as well as other benefits such as faster check-in, security, and other good stuff).
It's probably best for him to pick an airline/alliance based in his hometown, assuming that's where he always flies from/to.
One good option (not the only one) in that area might be Finnair (part of the Oneworld alliance), who have their hub in Helsinki.