Not all single speed bikes use the track standard 120mm but it is something you need to check - easiest to try and drop a “normal” wheel into the rear dropouts. If it is an actual track bike it’s unlikely to have mounting points for either a derailleur or a tensioner but you could fit a chainring tensioner.
Without actual details on the OP’s friend’s bike all this is guesswork 
B16 = Felt B16? https://www.wiggle.co.uk/felt-b16-105-2017-tt-bike