A booster is truly the worst solution for a "Houdini" kid. Unbuckling a seatbelt is much easier than unbuckling a harness--only one buckle to mess with! And a not-quite 3yo has absolutely no business in a booster. 4 years old and 40 pounds is the bare minimum a child should move to a booster, and most kids are not really mature enough to sit properly in a booster at all times until kindergarten-age (5-6). A booster will provide no protection if the child is leaning out of position or has unbuckled. Giving the escape artist child
more freedom in the car is not the solution.
Unfortunately it is mostly a discipline issue at this point. Being firm and consistent is the key. It is a phase that will pass eventually. At almost 3, I began showing my daughter crash test dummy videos of harnessed seats and boosters, and vehicle crash-testing. I explained to her in simple terms that her seat was to keep her safe, because if she wasn't in her harness she would be seriously hurt (as evidenced by the videos showing child crash test dummies flying all over the vehicle when improperly restrained).
This is definitely a tough issue, and I'm not trying to dismiss it or take it lightly, not at all. It's just that a booster is not an option for this child.
http://www.car-safety.org/faq.html#Q31
As an aside, have your friend check where her child's shoulders are in relation to the Titan's top slots. If shoulders are over the top slots the seat is outgrown, regardless of overall weight. Even if the child's shoulders are still below the top slots, at 35 pounds and not yet 3, it might be wise to begin researching higher weight harnessing seats.