In patients with low testosterone, testosterone treatment can improve libido and erectile dysfunction, but many men still may need additional oral medications such as sildenafil, vardenafil, or tadalafil. Some studies suggest that men with ED and low testosterone may respond better to PDE5 inhibitors when given testosterone therapy; however, this is controversial.
Stiffy Solution: Frustratingly enough, the only solution to exhaustion-based impotence is to get some rest, which is obviously difficult (or your dude wouldn't be having this problem in the first place). But if your guy has been resistant to getting help for his insomnia or asking for different hours at work, the inability to get his nine iron out on the putting green might be the thing that finally motivates him to make a life change. So, at least there's that.
Cavallini, G., Modenini, F., Vitali, G., & Koverech, A. (2005, November). Acetyl-L-carnitine plus propionyl-L-carnitine improve efficacy of sildenafil in treatment of erectile dysfunction after bilateral nerve-sparing radical retropubic prostatectomy. Urology, 66(5), 1080-5. Retrieved from http://www.sciencedirect.com/science/article/pii/S0090429505006515