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She's been cheating on her husband for years.
Literal
She [topic-は] how-many-years-even husband [object-を] has-been-betraying.
Two layers of time-marking work together. 何年も ('many years, several years') uses the 何 + counter + も pattern for vague-but-substantial quantity. The verbal complex 裏切ってきた pairs 裏切る ('betray') with ~てくる, the auxiliary that here means 'has been V-ing up to now' — actions extending from a past starting point through to the present. 夫を裏切る ('betray one's husband') in this context typically means 'cheat on one's husband' — the verb covers infidelity, deception, and broader breaches of trust; context picks the sense.