Unfortunately there are many factors that need to be taken into account. Based on the facts that you provided, it is unlikely that you would be charged with a crime. (I should note that finding any attorney that will guarantee anything to be legal is difficult.)
In addition, it would be illegal to record any sexual acts in any way. It would be illegal if it was nonconsentual. It may be illegal if you are in any position of authority over her (boss, manager, therapist, uncle, ect.) There are also several other factors that could cause this situation to be illegal, but you haven't indicated any of them.
I would note that "technically" having sex would be illegal under Minn. Stat. 609.34 Fornication. However, I have never seen prosecution actually try to charge someone under this statute, and I suspect that I never will.
This information is general in nature. You should not rely on this information as legal advice, as each case is unique. This information does not create an attorney-client relationship. If you are in need of legal advice concerning a particular matter, you are encouraged to contact us at your earliest convenience.