What are some typical lossless compression ratios?
It really depends on how much redundancy there is in the data. If all 250GB was just '0', then you could get fabulous levels of compression.

This page shows some results for compression of English text. It compresses 2,988,578 bytes worth of text using various techniques. The top three are: 330,571 (88.94%), 333,759 (88.83%), and 352,722 (88.20%).

Relating this back to your case: that means 250GB would compress (at best) to 27.65GB.

So perhaps there are lots of blank, repetitive patches in the data?



UPDATE:

Based on your edit, here is another paper that looks at XML compression rates.
The upshot (Figure 8; screenshot below) is that they appear to see compression rates ranging from 1 (none) to about 50 at a maximum.  That suggests that you are correct to be suspicious of your customer's 62.5 to 1 (250 to 4) compression rate.