"to get the boot in" - can you explain the idiom?

I think it's quite straightforward. To "get the boot in" is literally to kick someone (with BOOTED foot) when they are down on the floor. In your example, strangers metaphorically kick her at an early stage during their encounter to put her at a psychological disadvantage.

get the boot or bounce or can or heave-ho or hook or sack. Be discharged or fired, expelled, or rejected. For example, He got the ax at the end of the first week, or The manager was stunned when he got the boot himself, or We got the bounce in the first quarter, or The pitcher got the hook after one inning, or Bill finally gave his brother-in-law the sack. All but the last of these slangy expressions date from the 1870s and 1880s. They all have variations using give that mean "to fire or expel someone," as in Are they giving Ruth the ax? Get the ax alludes to the executioner's ax, and get the boot to literally booting or kicking someone out. Get the bounce alludes to being bounced out; get the can comes from the verb can, "to dismiss," perhaps alluding to being sealed in a container; get the heave-ho alludes to heave in the sense of lifting someone bodily, and get the hook is an allusion to a fishing hook. Get the sack, first recorded in 1825, probably came from French though it existed in Middle Dutch. The reference here is to a workman's sac ("bag") in which he carried his tools and which was given back to him when he was fired. Also see give someone the air.