OP - Just out of curiosity, how old are you? If that's being too nosy, you don't have to answer!

However, is this your first time being around a baby?
I agree w/ others that leaving a 7 month old baby alone & crying for an hour & 1/2 is just not good at all - for her emotional development, her sense of security, etc.
That said, like others have also said, there are worse things you could do than leaving a baby alone in his/her crib - if you're leaving the baby in her crib for your sanity & to avoid reacting to the baby's cries in frustration and anger, then, yes, you need to leave the baby in the crib.
However, if that's the case, then you need to discuss the situation w/ the baby's mother & tell her that you taking care of the baby for extended periods of time (like over an hour) is not going to work.
As a mother, I would not want my baby left alone in her crib crying, & my heart hurts for this baby.
Anyway, if you're honestly looking for ideas to help the baby not cry & to learn better ways in which to stop the crying...
Redirect! You said the baby will be playing & then come over to you, pull up, & start crying. Try to stop the crying before it starts. When you see she's getting fussy, pick her up for a moment & cuddle her to reassure her & then redirect to a fun new activity - something that she doesn't do all the time, so it has a level of "newness" for her.
Is she sitting up on her own yet? Put some water in a shallow plastic bin w/ some toys & let her sit in front of the bin & have some water play - DO NOT LEAVE HER ALONE W/ THE WATER!
Babies love to empty boxes. Give her a box of tissues & let her pull the tissues out. Put several small items (but NOT small enough to be choking hazards) in a box & let her empty the box. Don't make all the items toys - put some regular household items in the box - spatula, plastic cup, remote control, keys, brightly colored things, etc. The more "new-to-her" items, the more fun!
Give her some pots & pans & spoons from the kitchen.
At 7 months old, is she eating small finger foods yet? At the first sign of fussiness, give her a small bowl of Cheerios. There used to be a baby in our church nursery who was very fussy & would cry a lot for his mother, & I swear we'd keep him occupied for the entire hour just by feeding him little bits of Cheerios.
Put her in a small stroller & stroll her around the house.
Put some shaving cream or some other squishy kind of material in a gallon-size Ziplock bag - make sure it's closed & then let her squish it w/ her fingers.
If she's crying, try carrying her around to all the windows & letting her look outside.
Baby Einstein videos are also great. Also, while she's playing, have some background music playing to help maintain a calm environment - a kids' CD or classical music.
And, remember, at this age, babies don't really play by themselves and/or occupy themselves for very long - at least mine never did! And a baby's attention span is short too. So you can't expect the baby to play quietly by herself the entire time her mother is gone. You're going to have help her get started playing w/ an activity or watching a DVD or whatever & then be ready to redirect her to something else when she's done playing or starts to get fussy.