My advice is to tie both her hands together, and attach them above her head to the bed. That way, she can't get away but can turn over. It's only the tiniest bit scary.
Then, starting at her feet, he should very lightly caress her with his fingertips. Run them up and down her soles, under her toes, across her arches. If she isn't ticklish, the light caresses will be sensual. That way, either way they both enjoy it - if there's no tickling, it's still really fun.
Then he should go up her ankles, to her lower legs. Lift her legs a bit to caress the calves and under her knees - linger there a bit.
When he gets to the thighs, don't get too close yet to her inner thighs unless they are ticklish. It's more of a tease if he saves that for later...
Now for her neck. The sides of the neck are often very ticklish, and always very sensual. He should work his way down her arms, to her fingers, then back up to the armpit. Again, lingering a bit and brushing closely to her breasts.
Have her turn over. The back is a weak spot for so many people - and with her arms above her, and him sitting on her butt, she'll be helpless. After he's finished with her back (don't hurry), flip her back over again and tease her breasts and inner thighs. If he can't make her laugh, he should make her beg for an orgasm. Some women become very ticklish when they are extremely horny, so any promising spots he's discovered should be re-explored now that she is turned on.
Many people also become much more ticklish after an orgasm. Depending on the mood, this might also be fun to explore.
