Under what conditions is the oil being checked? If she's checking immediately after getting off the interstate, it might not be an accurate reading.
High speed driving can exacerbate oil losses. If the coolant leak is resolved and she's religious about checking the oil on the trip, I don't see why she couldn't continue.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13