While I have given many sensual and erotic massages some arraigned by the husband, I would never agree to what you are suggesting. She needs to have consistently found the fantasy of another man sexually massaging her to be both exciting and desirable. For a couple that finds the idea of a threesome exciting then a massage where the woman is more willing to be naked in front of the stranger and acceptable to be touched is often a good ground breaker. However, even then about a third of the time the woman is not comfortable enough to move on to full sexual activity.
When facing such a massage I explain that "her husband has arraigned a "full" body massage. I explain that in Europe, where I was first introduced to massage, there is no imaginary line between areas that are sexual and those that are not. Instead in her massage no area is off limits and yet there is no expectation that she is going to respond one way or another. This massage is simply about her pleasure, if it feels good we will do more of it, if it is not pleasurable or if it makes you uncomfortable we move on to some other area - I will simply follow your body's response and your verbal or non-verbal clues."
Most women become aroused, but that does not mean they will want further sexual contact. This is delicate and I warn husbands I will be fully in charge and no matter how excited you may be I will follow her lead, not yours. Also i think the secret recording is a bad idea. Most women respond much better, with the husband there if she is not fully in on what is going to happen. If she knows it may be sexual then her being alone is best.
If she enjoys the first time there will be many opportunities to record it later.
Personally it sounds like your fantasy not hers - that is usually a bad sign.