2018 NFHS Softball Rules bookThe umps here would know better obviously but I don't think this is breaking any rules unless the runner was carried home on his back...
Rule 3 Players Substitutes and Coaches
Section 5 Coaching
Art 4 A coach shall not physically assist a runner during playing action.
Out
Also Section 6 The Runner is Out
Art 5 Any offensive team member, other than another runner, physically assists the runner
Out
And it was the opposing coach that pointed it out. In a nutshell, you can get away with a high 5, but do not touch the runner. And contact can be then interpreted for assistance.
Believe me, we looked it up.