IAAL is absolutely correct in that 16 is the legal age of consent in Oklahoma.
However, other laws have been successful in cases like this, if you are considering possible legal action. And sometimes even the fact that you are aware of these laws and making him aware of them can sometimes 'solve the problem' in itself.
Some are:
1) 21 O.S. § 856 (OSCN 2001) - Delinquent Minor - Contributing to Delinquency:
"A. 1. Except as otherwise specifically provided by law, every person who shall knowingly or willfully cause, aid, abet or encourage a minor to be, to remain, or to become a delinquent child or a runaway child, upon conviction, shall, for the first offense, be guilty of a misdemeanor punishable by imprisonment in a county jail not to exceed one (1) year, or by a fine not to exceed One Thousand Dollars ($1,000.00), or by both such fine and imprisonment.
2. For purposes of prosecution under this subsection, a "runaway child" means an unemancipated minor who is voluntarily absent from the home without a compelling reason, without the consent of a custodial parent or other custodial adult and without the parent or other custodial adult's knowledge as to the child's whereabouts. "Compelling reason" means imminent danger from incest, a life-threatening situation, or equally traumatizing circumstance. A person aiding a runaway child pursuant to paragraph (4) of subsection (a) of Section 5 of Title 76 of the Oklahoma Statutes or aiding a child based upon a reasonable belief that the child is in physical, mental or emotional danger and with notice to the Department of Human Services or a local law enforcement agency of the location of the child within twelve (12) hours of aiding the child shall not be subject to prosecution under this section.
Source:
http://www.oscn.net/applications/oscn/deliverdocument.asp?citeID=69394
NOTE: I know, we have not heard of any issues of delinquincy in this case, but the above Code could apply if the circumstances warrant.
2) 21 O.S. § 1021 (OSCN 2001)- Indecent Exposure - Indecent Exhibitions - Obscene or Indecent Writings, Pictures, Etc. - Solicitation of Minors
I know 'another stretch', but it is common that these 'events' happen in a public location (19 and 16 year olds seldom get motel rooms).
Source:
http://www.oscn.net/applications/oscn/deliverdocument.asp?citeID=69510
3) 21 O.S. § 1123 (OSCN 2001) - Lewd or Indecent Proposals or Acts As to Child Under 16
Since you say that she just turned 16, I would have to assume that he did violate this state law.
Source:
http://www.oscn.net/applications/oscn/deliverdocument.asp?citeID=69615
Best of luck to you... you will need it in dealing with this problem.