You are on solid grounds asking her to cover her cleavage. Approach it something like this:
"We need to dress in a way that is not distracting to the customer. A shirt that shows too much of a lady's cleavage is distracting to the customer, not to mention the other employees. When too much of a lady's chest is on display, it's uncomfortable for others. The polite gentleman doesn't know where to look. The polite lady similarly doesn't want to appear to be staring. Thus, you need to wear shirts that cover your chest. Tight clothing, on anyone, is not appropriate in the workplace. You need to find some maternity pants or perhaps a dress that better suits your job."
"One potential problem is that someone could consider sexually distracting clothing as contributing to a hostile work environment. It creates a sexually charged environment where none should exist."
"Take this as a verbal warning. If you cannot or will not make a change, then in our at-will employment environment, this could be grounds for termination."
In my office, we had a similar situation. We are a conservative financial firm, and naturally expect people who want to work for us to "fit in" with clothing appropriate to a conservative financial firm. It was never a problem... until... one young man wore a sleeveless dress shirt to work! The shirt was styled just like any other male dress shirt, but was sleeveless. Thus, anyone coming near him got an eyeful of armpit hair.
The managers got together to say "Now, what do we do?". The upshot was that we did have to come up with a reasonable dress code. We batted ideas back and forth, anticipating what other issues might come up in the future. The proposed dress code was passed on to the employees for comments, then solidified. It is given to job candidates, with the unspoken message that "if you can't live with this, don't bother applying".
I suggest you, as office manager, do something similar. Come up with a defined dress code that suits your office, your climate, your customers, your industry and your business needs. You need to be comfortable, yet you need to look professional and keep peoples' eyes and minds on the job at hand, not someone's boobs and butt.