This is slightly under 2. Ever hear of the terrible twos? Things to be aware of are many. Children this age move very fast, get short tempered and scream and cry when things are taken from them, but also can choke on small objects. Turning your back for a minute can be disastrous. You have to keep the child from finding cleaning supplies and small objects. When I was your age I babysat one this age and he got into his own diaper changing supplies, covered his hair with vaseline and poured an entire bottle of baby powder on the floor. Basically don’t expect to read a book or watch a movie. You’re going to have to really watch a child this age closely, which is why the mother needs you even though she’s in the store.
You need to try to contain the child in a safe place, like a back room, and keep the child entertained. Since mother is there, if anything goes wrong you will be able to ask for help. Some toys say “not for children under three” because kids this age can choke on small objects. So if the kid picks up tiny things like a screw on the floor, you will have to the choke hazard away. The kid will then have a fit. Some children this age have drowned by leaning into mop buckets and falling in head first. Some children this age have died by being hung up in cords such as on blinds. They run in the street in front of cars if not held on to and they will try to drink windex or whatever they find. Your eyes are needed so the mother can work. It is great that she’s there though so you can ask for directions or help if it comes to that.
The problem is their legs start working before their brains do. :) You have a very important and busy job.
Sing songs, play games like peek-a-boo and patty cake, soak the kid with lots of attention and try to keep him distracted with age appropriate toys. Mostly keep him from sticking things in electrical outlets, pulling things down on his head, or eating anything that is not food.
Taking a friend is a bad idea because it will take your attention away from what you were hired to do. Most mothers are careful about who they would want around their kids anyway. Most likely she knows nothing about your friend. It can cause drama. Visit friends after work not during.