If you're going to build weapons for other people, friends or customers, for profit or for free, it's cheaper to pay for an 07 FFL, than to keep a lawyer on retainer.
In either case, I'd be more concerned about Civil Liability, than Criminal Prosecution for manufacturing without a license or tax evasion. You can get the proper licenses and pay the appropriate taxes. However, since you aren't actually making all the parts yourself, and in fact are using used and surplus parts, you may have little control over the final quality (read safety) of the product. Additionally, you have no control over the weapon once it leaves your possession.
Remember, "No good deed goes unpunished". It may reach your level of comfort if your "friend" wants to use your tools, under your supervision, to build his weapon, with his supplied parts. I'd take a hard look at what my homeowners or liability insurance would/wouldn't cover.
I guess my point is that Liability Insurance may be a necessity, even if profit is not a goal.
I'm neither a Lawyer nor an Accountant, so none of my comments should be taken as legal or accounting advice. I have, however gone to a school of great minds and allegedly know something about commerce. Additionally, I had type 06 and 07 FFLs, with SOT status, for six years and, at one point, was very adept at interpreting and dealing with the regulations of the BATF.
YMMV.