Not accounting for what the 'ethically correct' action is, this is a politically tough situation.
If the Liberals don't let him back in, it looks like an admission of guilt. If the Liberals do let him back in, it looks like they are uncaring towards the foreign interference issue.
They would have done themselves a lot of favours if they just 'temporarily suspended' him back when the news first dropped, and said his return is pending on the results of an investigation. At that point, it looks like a softer measure that could have been lower stakes that the party is entirely in control of.