It's not so big a difference to begin with.
According to what you say, Валюха or Валька are probably the most appropriate names (or so I think). Just Валя will do too, but Валечка or Валюша are too intimate to be used in such circumstances. Later, when their relationship progresses they would become appropriate.
P.S. And, if you don't mind my criticism, I can hardly believe that this model of relationships (more like brother and sister) could lead to any romance.![]()