I think Grace fits the bill perfectly. While the food and presentations are much more sophisticated and complex than they have previously eaten, it still "seems like food" and should be comfortable for them on such a special night. What tips the scales way in Grace's favor to me is the service. Everyone there is extremely professional, attentive, respectful, and gracious (in that regard, and if you have read Curtis Duffy's story, that description is intentional). Choosing wines by the glass or bottle instead of the pairings, or asking to share one pairing between the two of them, would be a way to lessen the cost, and he could work this out with the sommelier in advance of the meal if he did not want to discuss budget, preferences, and other details once the meal begins and "his lady" is present.
In my experience, tasting menus, even at venues I love, sometimes feel more like theater, where I am the audience rather than the star. I enjoy this type of show, with a lengthy cast of food and details of every dish. I bring this up, however, because I think for a special occasion, some places do a better job than others at pacing and adjusting service based on the perceived needs of the guests, especially on a first visit, and helping to facilitate the celebration beyond just presenting the stellar food. The two tasting-menu-only places in town I have been where I feel the staff does the best job of this are Grace and Goosefoot.
Edited to note: I just realized my response is on the Spiaggia board, so if mods think my post or this thread needs to be moved to a separate thread about proposal venues, or Grace, I understand. I purposefully did not mention Spiaggia in my post because I thought the OP had already decided against it, and I was not going to discourage this decision. Suffice it to say, in my two visits to Spiaggia, one on my birthday, I was not treated by staff in the manner in which I would want for someone's proposal.