Heaven is real, along with Jesus, angels and the souls of those who have died in Christ ... but the “stories” of some people going there and coming back here to “tell the tale” are not real.
The bible speaks of Paul having been caught up into some region of heaven, so we can certainly accept that such a visit is possible in principle.
I do not know much about the Malarkey story or his past or current attitudes towards the Lord and it would be foolish to assume anything without researching it. Miracles were not enough to persuade some people, full of themselves, to believe, and neither would be a visit to heaven. Satan was in heaven once for all the good that did in his case.