I agree with Maurice, and I think the reversion in the case of hostas is due to the variegated foliage being a sport of the parent plant's foliage. So, for example, the variegated "offspring" is not really an offspring of a parent hosta, but an unstable mutation that can revert back to producing the original color of its leaves. I'm not aware of any documented cases of daylilies producing flower-sports, though it may be possible as in other flowering plants. So, we don't know at this point whether daylilies may, even rarely, create sport's that might revert to a previous flower form.
Other known reasons to consider why daylilies might look like they have changed:
Ploidy: One type of "reversion" that daylilies have been documented doing is where the daylily has been converted to a tetraploid form, and later converts spontaneously back to diploid form, as Lin had found, though this would not likely result in the plant losing its own basic characteristics and picking up or losing different parental colors. It is just the loss of "extra copies" of their own unique genetic material rather than a change in their unique genetic material.
Underground Rhizomes and Natural cross-pollination: Daylilies can appear to revert back to older-type or species plants when pollinated by natural means, if the seeds drop and grow into new offspring (they may have some characteristics that resemble a cultivar father back in the line). And, many daylilies can self-propagate quite easily by rhizomes that tunnel underground some distance away and pop up right in the middle of another cultivar, also possibly crowding it out.
Years to First Bloom: Daylilies often need to grow for a number of years before they flower for the first time, so new fans can appear without showing themselves to be from a rhizome that has traveled some distance from a different cultivar (or may have arrived hidden within a newly purchased set of fans), or may even be a mis-identified daylily to start with. In that way, it might seem as though the original clump has suddenly changed its appearance once those "tunneled" fans are ready to bloom. Many daylily cultivars propagate this way.
Settling In: Once a daylily begins to produce blooms, they may look very different initially from what they will become. Over the span of three years many daylilies need to "settle in." For example, an expected double bloom may appear as single blooms, or a purple patterned bloom may appear light lavender with no pattern, or an expected large bloom may start out small. Usually by three years of blooming time the flowers will stabilize in color, size and form.
Heat and Sun Exposure: The daylilies that I have growing in shade grow almost twice as large and tall as the same variety that is grown in full sun. Color can be quite different depending on location, as can be patterns or other characteristics (for example, "teeth" or "broken eye patterns"). In some cases those characteristics may never show up (climate too hot or too cold).
It might also help to ask your friend some more questions, to understand the core nature of the question they are asking. Are they growing both a parent and offspring daylily in their garden, so that rhizomes and proximity may be mixing the two together? Are they seeing a daylily that has been stressed in some way take on a different look (possibly quite temporary, or permanent if new soil or climate conditions are introduced)? Is it possible they have had some seed pods release seeds into their garden? Or, are they just wondering if daylilies that look different from their parent plant are sports instead of unique genetic crosses between two sets of chromosomes? If you'd like to detail more background or specifics, there may be additional information we could share.
