C. F. Keil and Franz Delitzsch on Proverbs 17:18

Pro 17:18 18 A man void of understanding is he who striketh hands,      Who becometh surety with his neighbour.

Cf. Pro 6:1-5, where the warning against suretyship is given at large, and the reasons for it are adduced. It is incorrect to translate (Gesen., Hitzig, and others) לפני רעהוּ, with the lxx, Jerome, the Syr., Targ., and Luther, “for his neighbour;” to become surety for any one is ערב ל, Pro 6:1, or, with the object. accus. Pro 11:15, another suitable prep. is בּעד; but לפני never means pro (ὑπέρ), for at 1Sa 1:16 it means “to the person,” and 2Sa 3:31, “before Abner’s corpse (bier).” רעהוּ is thus here the person with whom the suretyship is entered into; he can be called the רע of him who gives bail, so much the more as the reception of the bail supposes that both are well known to each other. Here also Fleischer rightly translates: apud alterum (sc. creditorem pro debitore).