1
general::
Phrase(s): let something ride
Fig. to allow something to continue or remain as it is. • It isn’t the best plan, but we’ll let it ride. • I disagree with you, but I’ll let it ride.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, let slide. Allow something to be ignored or to take or continue in its natural course. For example, Bill disagreed with Mary's description, but he let it ride, or He had a way of letting things slide. The first term, alluding to things moving along as though they were riding a horse or vehicle, dates from the early 1900s; the variant, using slide in the sense of "pass by," dates from the late 1500s. Also see under LET SLIP.
American Heritage Idioms