BSF and Shore Trooper are both incredibly versatile. BSF will boost any sith lead she's with, so she's great under both Dooku and Vader. She has good synergies with Dooku in particular, as he also has the shock debuff. She will even function as a de-facto tank in a Dooku lead Separatist team.
Shore Trooper is incredibly tough and has a pretaunt ability, so he will work well in any team, but is probably best in that Vader team.
Otherwise I think your friend should work on unlocking Palpatine and Thrawn to make a strong empire team, and generally building up Separatists. Jango Fett and Asajj Vemtress are two very good Separatists that remain good throughout the game, and are also essential parts of BH and NS teams later on as he unlocks more Separatists, so those two should be a priority.
Why are you posting on your friend's behalf anyway?