I do stand corrected, Paul does cite his friendship with Young as a primary factor for the endorsement. However, Paul does support his endorsement with policy specifics on environmental and tax policy, so the friendship is, at least ostensibly, not the sole factor at work.
While Paul should be faulted for endorsing big-government Republicans in contested races, rather than merely staying neutral, I find it interesting that the same people who fault Paul for being "ineffectual" also criticize his Faustian pact with Republican officeholders to avoid perpetual primary challenges during his tenure.