There are so many great restaurants around Lausanne that it's difficult to choose when it comes to a special occasion! I'd add the Pont de Brent, L'Heremitage at Clarens, the Cerf at Cossonay and Ravet's place at Vufflens le Chateau to the list. For good, but not stellar, gastronomic experiences in a lovely setting, the Hotel ds Trois Couronnes and the Hotel du Lac in Vevey are worth considering. Denis Martin (the Chateau at Vevey) is great fun (in the Heston Blumenthal mode) but perhaps not for the older or more traditional diner......but if your mother has an open mind and a sense of humour you could consider it - prices, as with all gastro restaurants in the region, around 200/head plus wine.
Regarding Le Deck (the outside part of the Baron Tavernier at Chexbres), this is without a doubt the best terrase on the whole Léman but it is therefore very popular. The clientele is mainly young and well-off or groups of business people. This means that the enclosed restaurant section can be very noisy and I would certainly hesitate to take my parents there to eat, although I'd strongly recommend taking them for an apéro on the Deck itself. But even for that you should reserve ahead of time.
Whereever you end up I hope your mother has a wonderful birthday!