We should refuse to be unfaithful to our spouse even if the divorce is being processed in court.
Aside from that, the biggest disadvantage of dating too soon (assuming the person is legally divorced now) is entanglement.
I don’t discourage marriage but I also don’t put the pressure on people to get married.
The innocent new friend can be deposed by the other side’s lawyer (that is, asked questions that are taken under oath and recorded by a stenographer or even videotaped) and subpoenaed to testify at trial.
Divorcing clients are often lonely and stressed out, and they may be longing to meet someone new, feel desirable again, and just have fun.