You say you're adults, but that she is a girl.
Any chance you could just tell her mother? How does she know DH? Because I was wondering if there is another adult who could intercede - if he is a teacher he could be on very thin ice so talking to someone on SMT could be worthwhile, or confiding in a colleague if there is any way his occupation could be described as being in a position of influence over her.
The text to a family member has tipped this from girlish crush into circumstances under which your DH really needs to take steps to protect his reputation. MIL must be told not to answer texts, but to keep then, just in case there's an investigation.
If there are sources of advice open to DH, he should consult them before any further action (don't forget unions can also advise, or professional bodies if the girl knows him from a sport or other national club).
If it's not linked to his occupation/activity, then it's less likely to be career damaging or criminal (as it's only position of influence that lead to the possibility if prosecutions if the girl/boy is over 16) but could still be reputation damaging. So he really does need to respond to all overtures (whether spoken or in writing) by saying he's not interested. I would say do it as kindly as possible, given that she is so much younger, but as this seems to have been running a while, then I think there's a risk that any kindness could be seen as encouragement, so it needs to be firm now.