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Due to an anomaly with the second relight of the Merlin engine on a Falcon 9 Starlink mission, the FAA has grounded the falcon 9 and will conduct a mishap investigation. Can falcon heavy be used as a replacement for the time being?

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No. They use the same second stage, and the second stage is what failed:

Falcon 9’s second stage performed its first burn nominally, however a liquid oxygen leak developed on the second stage. After a planned relight of the upper stage engine to raise perigee – or the lowest point of orbit – the Merlin Vacuum engine experienced an anomaly and was unable to complete its second burn. Although the stage survived and still deployed the satellites, it did not successfully circularize its orbit, but it did passivate itself as normally performed at the end of each mission. This left the satellites in an eccentric orbit with a very low perigee of 135 km, which is less than half the expected perigee altitude.

I briefly thought that maybe single-burn missions could (but likely wouldn't) be allowed, but Jörg W Mittag noted that relights are necessary for controlled deorbit, so despite SpaceX and the FAA's relatively cavalier attitude toward dropping large pieces of spacecraft wherever the second stage can't be permitted to fly until this issue is resolved. ...unless they're escape-trajectory missions? Still probably not.

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    $\begingroup$ If I understand correctly, the FAA doesn't allow the second stage to be relit until after the investigation. This means it doesn't matter whether they use the Falcon 9 or Falcon Heavy, as both use the same second stage. $\endgroup$
    – The Rocket fan
    Commented Jul 14 at 20:50
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    $\begingroup$ A relight is also required for a controlled deorbit. $\endgroup$
    – Jörg W Mittag
    Commented Jul 14 at 21:03
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    $\begingroup$ I would agree probably not, until the cause is found. Depending on what caused the leak there is no guarantee that a reoccurrence will not lose even more propellant and possibly severely affect or cause the loss of a payload even on a mission that does not require relight. $\endgroup$
    – Steve Pemberton
    Commented Jul 15 at 3:04

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