Attacking a target behind another creature grants the target half-cover (PHB 196). It might grant more. Half-cover grants the target +2 AC.Sorry, but im not too familiar with the -5 penalty from Darkvison and the +2 AC penalty for shooting at enemies fighting allies...in what book is this found?
For theater of the mind games, +2 AC to the target for shooting into melee is a good compromise. Technically this should only apply if your ally (or some other creature) is between you and the target, but TotM games are usually looking for shortcuts and characters in a melee would be moving around a lot, so assuming half-cover is generally seen as fair, believable, and simple enough to keep things moving. Although 'hitting an ally on a nat 1' is another popular option.