[glassmaker]

Is the bottle glass compatible? Probably. There isn't a huge range of composition in typical, contemporary bottle glasses.

Is the statement she made about laying the glasses side by side accurate? Maybe.

You asked if the two glasses were compatible. They may be compatible enough to work the way she is using them, but as she stated, not good enough to work if one is encased with the other. Glasses with differing COEs and strain points can be made to work together depending on how they are used. Side by side is probably the most benign way to join them. Encasing one with another is definitely not.

I have a little bit of experience with this. Back in my scientific glassblowing days I sometimes had to make graded seals to join 33 COE boro to 90 COE soft glass. The transition between the two is typically made with several intermediate glasses in between. Done right, the pieces hold together just fine.

Two standard bottle glasses are likely to be closer in compatibility than any of our transition glasses were. But since you can't test the glass yourself, you can't know for sure how close - or far off - they might be, other than taking a look at the piece using a polariscope. So I agree with Phill - if you like the piece and it wasn't terribly expensive, go for it. If the cost is too high to risk, pass it by.