The mortgage interest supplement would be affected.
Her additional income is assessed as -
first 75 euro of wages are disregarded
1/4 of the remainder (125) is disregarded = 31.25
Her means from wages are calculated as 200 - 75 - 31.25 = 106.25. The fact that her OPFP has been reduced would be taken into account, so her Mortgage Supplement would be reduced by 106.25 - 27 (the OPFP means).
If she worked more than 29 hrs per week, her entitlement to Mortgage Interest Supplement would cease as she's considered in full time employment.