Knowing nothing about other traits the plane has I agree that it sounds like too much down thrust.
Although since you are talking about a biplane, that involves a whole new set of problems, such as incidence of either wing or ailerons setting between the top and bottom wing being different.

After checking the thrust angle use Pete Goldsmith’s trimming guide. http://www.flagstaffflyers.com/image...thTrimming.pdf

P.S. When your friend finds the cause please post to help other modelers.