Just trying to bring someone round to the idea of getting messy. She sounds like she's up for a pie or 2 to the face, so need to find a short clip or video showing her that it can be fun, or amusing for her, without it coming across as creepy and having a full 10 minute session. If one could involve maybe a pieing followed by a gunging too, or vice versa would be good.
Not meaning to brag but I saw one of the models who shot with us at Easter for www.messymayhem.com and she asked me if she could do it again and described it as "one of the most fun things I've ever done". Many models have commented how fun our shoots are. I do my best to make it fun for the girls with silly games etc because if they are enjoying it the shoot results are better. Check out the site and see if you find anything that fits you bill. I think you definitely have the right approach by thinking along the lines of fun. If you went straight in for handcuffing her and covering her in 5 gallons of beans for example that could leave mental scarring!
I have been told many times over the years that my vids were the "gateway drug" for folks to introduce their partners to WAM.
Take that as you will, but I consider that a huge compliment.
I'd recommend the videos where the models are more "girl next door" than "MAXIM goddess next door"... Or the ones where the girls are very goofy or always cracking up. (The 3 Southern girls in SS58 are good... Brit from SS100/105 is good... Shelly is always good...)