This is sometimes the hardest part of being an employer. If an employee who has been with you for years, or even decades as a trusted or loyal employee is accused of doing something wrong, your natural instinct may be to stand by them and support them. While this may be mistaken for loyalty, choosing sides without a full assessment of the situation can be a dangerous move.
Take a pause to assess the situation and the allegations in full. If no legal claim has been commenced, this may be the perfect opportunity to conduct a small investigation, similar to one that would be conducted for an allegation of harassment. Make the effort to speak with the person who made the allegation to learn their story, and allow the subject of the allegations a chance to answer for their behaviour.
As a final note, the investigation must be conducted by someone objective in order to be properly effective. If a long-service employee is investigated by a manager who coincidentally happens to be a friend and colleague of theirs for decades, it is almost guaranteed that the complainant will claim that the investigation was biased. In these situations a third-party investigator can be a worthwhile investment.