I do personally feel most comfortable with just tackling these kinds of things (expectations I don't feel I can live up to) before the dinner in a truthful manner, but as you've ruled out telling Alice the true reason for not wanting to share: I'd like to introduce you and your friend to the wonderful concept of pro-social deception. These are lies that are designed to benefit the person lied to or lied about, sometimes also just called 'white lies'.
As your friend's goal seems to be to protect her friend from an unpleasant suspicion, a pro-social deception will likely save enough time until the next dinner (if there will ever be one, you say it probably is a one-off situation), time that can be used to find out if Alice really has something that might be infectious. I've had dinners where people that had a cold were double-dipping or sick brothers that don't wash their hands and then grab food from a container. A white lie about not liking that particular dip, or not feeling like eating X or Y often worked fine to avoid sharing that food.
While lying often is seen as bad and harmful, there is some scientific evidence that suggests that pro-social deceptions can actually be beneficial to some forms of trust:
Intentions are more important than honesty for building benevolence-based trust.
So, a pro-social deception, even if found out, can still make Alice trust that your friend has good intentions towards her. On the other hand, that same scientific article concludes that Alice might start doubting your friends integrity:
Although prosocial lies increase benevolence-based trust, they harm integrity-based trust.
Meaning that next time Alice asks your friend if a dress looks good on her, and your friend answers with yes, Alice might think twice before taking that as an answer: She might start wondering if you're just being kind or really being honest.
So, your friend is going to have to think this through, and make sure she really wants to do this! To help, this blog post presents a little checklist with questions she can answer for herself, to determine if a pro-social deception might be appropriate, such as whether or not it would be kinder to tell the truth or lie, whether the lie is likely to be found out, and at the bottom (but most important to me) whether your friend would mind if Alice told her a lie in a similar situation.
On a side note, another thing that I sometimes use to avoid having to share: Order the same thing as the other person. This only works if you like what that person is ordering, and they order first, but it sometimes is an option!
