The England prop was in his first game back after the two-match ban for his “Gypsy boy” slur against Wales tighthead Samson Lee during the Six Nations.
The 25-year-old’s kick to Heguy in the 26th minute of Quins’ semi-final was not picked up by referee John Lacey, but Marler could still face a citing and a disciplinary hearing.
Marler initially escaped censure for his taunts against Lee in England’s 25-21 Six Nations victory over Wales at Twickenham on March 12.
Global governing body World Rugby later intervened and called a misconduct hearing however, dissatisfied with the leniency of Six Nations and RFU bosses.
Marler admitted the charge and was banned for two matches and fined £20,000.
Jamie Roberts, the fit-again fly-half Nick Evans and George Lowe claimed the tries that sent Quins into their first European final since winning the Challenge Cup in 2011.
Conor O’Shea’s men will now face the winners of today’s other semi-final clash, between Montpellier and Newport Dragons.
Harlequins’ 13-6 half-time lead was not nearly sufficient, by virtue of both their early dominance and Grenoble’s threats of a recovery.
No one of a French persuasion much fancied stopping juggernaut centre Roberts in fifth gear, and the Wales midfielder duly trucked over for the first-half’s sole try.
Ben Botica’s conversion and two penalties had Harlequins in control, but Jonathan Wisniewski was able to land two goals of his own to cut the home advantage.
Quins were 13-3 to the good just inside the first quarter.
Whatever the reasons, Grenoble created enough of a scrap to scrabble a foothold.
Three times though the visitors set themselves a fine platform, only to botch their approach play.
Quins somehow escaped and were able to close out the clash after the break despite yet more fluctuating fortunes.
Fit-again fly-half Evans capped his return from a broken leg with the try that effectively sealed Quins’ victory, the Kiwi playmaker reclaiming his own grubber to score just ahead of the hour.
Danny Care sent a grubber of his own through next, allowing centre Lowe to dot down Quins’ third and final try.