Lanham-New et al.1 compared the literature for bone health in ovo-lacto-vegetarians and vegan diets against omnivores with predominantly meat diets. They found no difference in bone-health indices between those two groups.
This review indicates that the source of the dietary protein does not make a significant difference for bone health.
The claim that animal proteins increase the acidity of the blood is also not credible. The pH value of the blood and inside cells is tightly regulated2 and not easily influenced by diet. The content of the stomach there is already a very acidic environment, which shows that the pH can be effectively regulated by the body.
There are also several studies indicating that milk consumption has a positive effect on bone health and decreases the risk of osteoporosis:
Effects of calcium intake, milk and dairy product intake, and blood vitamin D level on osteoporosis risk in Korean adults: analysis of the 2008 and 2009 Korea National Health and Nutrition Examination Survey:
The results of this study showed that BMD increases as Ca intakes
increases, and that the risk of osteoporosis incidence was reduced
when more than 1 serving of milk or dairy products were consumed and
serum 25(OH) vitamin D level was maintained in the normal range
Calcium, dairy products and osteoporosis:
Of 52 investigator-controlled calcium intervention studies, all but
two showed better bone balance at high intakes, or greater bone gain
during growth, or reduced bone loss in the elderly, or reduced
fracture risk. [...] While most of the investigator-controlled studies
used calcium supplements, six used dairy sources of calcium; all were
positive. Most of the observational studies were based on dairy
calcium also, since at the time the studies were done, higher calcium
intakes meant higher dairy intakes.
 S.A. Lanham-Newa, W.T.K. Leea, D.J. Torgersonb and D.J. Millwarda, Is vegetable protein more beneficial to bone than animal protein?, International Congress Series 1297, Nutritional Aspects of Osteoporosis 2006. Proceedings of the 6th International Symposium on Nutritional Aspects of Osteoporosis (2006)
 Lodish et al., Molecular Cell Biology. 4th edition, New York: W. H. Freeman (2000).