Single member LLC's are treated like sole proprietors by the IRS and the activity for the business is included on her (or your joint) personal tax return on Schedule C, E or F (depending upon the type of business). There is not a separate return done for the single member LLC. If there was no activity for the business during 2018, then you do not need to include it anything for it on your return. See more information from the IRS below:
If a single-member LLC does not elect to be treated as a corporation, the LLC is a "disregarded entity," and the LLC's activities should be reflected on its owner's federal tax return. If the owner is an individual, the activities of the LLC will generally be reflected on:
An individual owner of a single-member LLC that operates a trade or business is subject to the tax on net earnings from self employment in the same manner as a sole proprietorship. Single Member Limited Liability Companies | Internal Revenue Service
Also, see this per the IRS:
If your sole proprietorship business has no profit or loss during the full year, it's not necessary to file a Schedule C (Form 1040), Profit or Loss from Business (Sole Proprietorship) for that year. Schedule C & Schedule SE 2 | Internal Revenue Service - IRS.gov
