Yes - we have experience of this - ours is termed Benign Hypermobility Syndrome (other name Ehlers-Danlos Hypermobility Type (also known as Type 3) - my two children inherited it from me. Look at www.hypermobility.org to see if this sounds like your family.
The doctor sounds right - we have all been slow to learn to walk but you do get to walk eg 2 yrs old for my son, 19 mths for my daughter (both took longer to reach each milestone and needed physio, occupational therapy and orthotic boots to help them along their way).
We do not feel too much pain but get fatigued, bruise easily, look a bit clumsy (some children are mistakingly diagnosed as dyspraxic). Some hypermobile people are fab at gymnastics and ballet (also some famous footballers, cricketers and tennis players have it too) but we are not so sporty.
Perhaps start asking in your family about any clumsy people and see how they are as an idea how it can be for your family. Were any late walkers?
Thoroughly recommend the Hypermobility Clinics listed at the website.
Surgery is often the last resort after physio - maybe your friend should seek advice from a Hypermobility Clinic before she has the big op?
Usually children sit in W on the floor - ie feet either side of hips. Crawling is later and can of often be bottom shuffling as support in arms is not as strong due to the weaker collagen (connective tissue) structure in ligaments, muscle and skin.
My two have worn orthotic boots (which look fab and often people say ohh where can I buy those?) since they were babies learning to stand as boots give stability to ankles to help you stand =- ie before cruising). Later they were given inserts as well as boots - I have these too and feel better supported. Is anyone in your family flat footed or do they have high arches - do they have bunions - all hypermobility issues. I wish I could have had boots when learning to walk as I took till 3 yrs old to walk. Days are much different now.
For me, sprains occured in teenage years esp knees and jaw but everyone is different. No sprains yet for my two (eldest is 12).
Do ask anymore questions if you like.