Frost on suction line

At a friends house the other day and checked on an 15kW R22 split sytem unit with capillary expansion, unit is approx 10 years old with frost on suction line right up to compressor, unit operating on heating, all fans running and some heat coming from ceiling vents, filters are clean, outdoor temp 8*C, indoor 17*C, air from ceiling vent 26*C, did not have gauges with me. Frost build up starts to occur after only operating for a few minutes operation, complaint was unit running for longer time with reduced heating, looks like unit under charged, any other reasons that could cause this frost to occur?

Not enough air passing through the evaporator (outdoor coil)?

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There was no visible frost on the evaporator at the time the suction line was frosting up and although I had no air flow meter there appeared to be sufficient air movement over the coil.

Sounds like over charged system and refrigerant boils in the suction line.

Unit has been operating for several years without any obvious problems and has not had any service work done (as advised)
Is it possible that there is a restriction in the capillary or distributor tubes to the evaporator (outdoor) coil.
Think I am going to have get back to the unit and take more readings.

Double check the external heat exchanger (evaporator) including the airflow of the fan and clean the battery.

Certainly will be checking airflows but what is "clean the battery".
If relating to temps taken these were via digitemp themometer and were checked a number of times.

by battery might be referring to coil