Normally a synopsis should be entirely in present tense.
But there is one place where you might have to go into past tense to keep your reader oriented.
Some synopses consist of character A going along, doing his or her thing. Then the synopsis goes like this: Meanwhile, character B did this and is now doing that. The purpose is to let the reader now we need to move back in time so that we're looking at something that B was doing at the same time A was doing something that we know occurred earlier in the book.
This is going to be real quick, no more than a sentence, and maybe not even the entire sentence. The problem if you don't use the past tense is that the reader has a problem staying oriented in time as to what is happening when.
One of the fun things about queries and synopses is that there are all sorts of rules, and then there are the exceptions.
ETA: And here you hit one of the exceptions to the rules. Memoirs are their own little beasties. Memoir synopses tend to work best in first person, past tense. That way you tend to bring in the most personal involvement with the reader.
Best of luck,
Jim Clark-Dawe