A way of describing the lesbian parent that is not pregnant?

Using the inverted-gender pronoun for the partner in a homosexual relationship who is not physically pregnant is entirely sensible, although a bit odd.

If you want an alternative, parent-to-be is a fair term which is not mismatched on gender and does not include the same health restrictions as mother-to-be. Of course, one wonders how you would describe a woman who is expecting a child through a surrogate.

The non-birthing part of a lesbian relationship having a child is often called the co-mother (last sense—ignore the previous senses, they're very rare in normal settings, at least in my experience).

So your friend would be a co-mother-to-be or (perhaps less likely to make you suffer a hyphen overdose) expecting co-mother.

Perhaps something mildly humorous might do the trick. For instance:

"We're pregnant, but I'm still allowed to drink and go bungee-jumping".

"We decided that as I was better at rugby than her, she'd be the one taking a break".

"It turns out I'm having to drink for two".

@DougM's answer of using the gender-inverted ("father-to-be") or gender-neutral ("parent-to-be") terms is a good one for general-purpose, quasi-impersonal descriptions -- such as when you are describing your friend to us. "Mother-to-be", despite the connotations, is still the most technically accurate moniker, however.

When she's asked, or it otherwise comes up for her, she generally has far more control over the structure of her response and thus can take a page from my own friend's book: "My wife* is pregnant." (Substitute "spouse", "SO", "girlfriend", or whatever other moniker she/they prefer.) Since our society is still very heavily hetero-normative, a woman saying she's a "parent-to-be" is still likely to be heard in the exact same light as if she had said she's a "mother-to-be", i.e. it will be too frequently assumed that she's the one who's pregnant. Using "father-to-be" is just likely to sow confusion.

*Note: My friend in this case is not actually married to her SO, as they live in a state that does not (did not? haven't been following progress on that front too closely) recognize homosexual marriage yet, however they think of themselves as being married and thus use those terms (well, term: "wife") in all except legal settings. Which just goes to show that the labels you choose for yourself really are entirely up to you, so to bring this back around to the question at hand: Your friend should use whatever description or moniker she feels comfortable with, regardless of what a hetero-normative society that all too frequently still despises her may think about it.