BKS is correct for very formal occasions such as Her Majesty's Garden Parties, but this is not the occasion.
Hats are no longer required for ladies in Her Majesty's presence, nor gloves. Indeed, over-dressing on a less formal occasion is seen as terribly middle-class and non-U.
Smart attire appropriate for the occasion is the way (if outdoors, welly-boots and smock may be too casual, though the Duchess of Devonshire is famous for her disregard in this matter, and your mother wouldn't want to get mistaken for her, trust me).
If your mother likes hats and wears them well, a low-key choice may be appropriate. I would advise to avoid however, as she will appear arriviste.
As econ21 noted, a good guide is that she should dress in the manner she might adopt if the president were coming to dinner or a outdoor event, but not over-dress. Garish colours are not a good idea, of course.
If she is introduced to Her Majesty, it is permissible these days for women to make a small bow from the neck rather than curtsey. The latter is an acquired skill that can make one self-conscious and look foolish in a stressful situation. Best to bow, take the hand if offered (rare) smile and answer questions if posed. Speak only if spoken too. Her Majesty is extraordinarily good at putting people at their ease, so don't worry. Smile and enjoy. Do not ask for an autograph.
The rules are very much more relaxed than they used to be. Your mother should not worry unduly.
econ21, Helen Mirren's portrayal of the Queen was damn near spot on. I've been hunting at Balmoral and the picnics portrayed are just right. Lady Ghost was appalled that Her Majesty served herself sandwiches etc, but then she's foreign and the Russian aristocracy are big on servants.
If the invitation does mention dress code, feel free to come back to us (or PM) with details and we can advise.
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