It's more likely that there are a couple of factors at play here.
From babies children seem to have a 'will of their own' when it comes to development. With little stimulation they will be curious and move through the developmental stages (learning to roll, move around, make sounds, etc), if at a slower pace.
If this child does VERY little, she has either been very seriously neglected, or does have some developmental issues/learning diffculties (or whatever, that requires diagnosis). Most likely, it's a little bit of both.
Do you see this mother and baby often? And for how long? When poeple have visitors, you won't see the true picture or how things usually are in the home. Perhaps the child isn't awlays in the bouncer in front of the TV and perhaps they eat other foods, not just milk, etc. It might not be as bad as you think. Be careful that you're not making assumptions.
The child is obviously behind in their development. Whilst it's right that it's the responsibility of other bodies to intervene, it should also be the shared responsibility of all adults to keep our eyes peeled and ears open when it comes to cases where neglect may be occurring.
Try to keep in touch with and see this Mum regularly, bu don't ask her lots of specific questions as she may back away from you if she suspects you are monitoring her parenting. Keep your eyes and ears open. You can't do anything about the situation unless there is evidence of serious neglect (you witness neglectful behaviour - which is more than just noticing trends when you visit occasionally). If you see something serious you may need to notify someone - perhaps come back on here and get others' advice first. What you've described so far isn't necessarily a sign of serious neglect (just bad, lazy parenting). It was probably picked up at the child's 1 and 2 year NHS review anyway.